Explosives are a core component of nuclear, biological, chemical and conventional weapons, as well as of terrorist devices such as car, luggage, and letter bombs. Current methods for detecting the presence of explosives, include vapor detection, bulk detection, and tagging. However, these methods have significant difficulties dependent upon the nature of the signature that is detected. See, Fetterolf et al., Portable Instrumentation: New Weapons in the War Against Drugs and Terrorism," Proc. SPIE 2092 (1993) 40, Yinon and Zitrin, in Modern Methods and Applications in Analysis of Explosions, (Wiley, New York, 1993) Chap. 6; and references therein. Vapor detection is achieved using trained animals, gas chromatography, ion mobility mass spectrometry, and bioluminescence, as examples. All of these techniques suffer from the inherently low vapor pressures of most explosives. Bulk detection of explosives may be performed using x-ray imaging, which cannot detect the explosives themselves, but rather detects metallic device components. Another method for bulk detection involves using energetic x-rays to activate nitrogen atoms in the explosives, thereby generating positrons which are detected. This technique requires an x-ray generator and a minimum of several hundred grams of explosives. Bulk detection is also accomplished using thermal neutron activation, which requires a source of neutrons and a .gamma.-radiation detector. Thus, bulk detection is not sensitive to trace quantities of explosives and requires large, expensive instrumentation. Tagging requires that all explosives be tagged with, for example, an easily detected vapor. However, since tagging is not mandatory in the United States, this procedure is clearly not reliable. It turns out that none of the instruments described can perform accurate, real-time (&lt;6 sec) detection and analysis of trace explosives in situ except for trained dogs.
It is known that surfaces in contact with explosives (for example, during storage, handling, or device fabrication) will readily become contaminated with explosive particulates, as a result of their inherent stickiness. This phenomenon is illustrated in studies that show large persistence of explosives on hands, even after several washings (J. D. Twibell et al., "Transfer of Nitroglycerine to Hands During Contact with Commercial Explosives," J. Forensic Science 27 (1982) 783; J. D. Twibell et al., "The persistence of Military Explosives on Hands," J. Forensic Science 29 (1984) 284). Furthermore, cross contamination, in which a secondary surface is contaminated by contact with a contaminated primary surface, can also readily occur. For example, a measurable amount of ammonium nitrate (AN) residue has been found on the lease documents for a rental trucks, and significant amounts of the explosives PETN (pentaerythritol tetranitrate) and/or AN have been found on clothing and inside vehicles of suspects in two well-publicized bombings. Therefore, explosive residue will likely persist in large amounts on the explosive packaging and environs, as well as on the individuals involved in building the explosive device, which provides an avenue for detection of the presence of explosives.
Accordingly, it is an object of the present invention to provide a detection and monitoring system capable of quantitatively detecting submicrogram and larger quantities of explosive materials.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. Key features of useful explosives detectors include high sensitivity and accuracy, substantial specificity, simplicity of operation, reliability, real-time measurement and analysis capability, low cost, and portability.